The present invention relates to a method and a device defined below in the introductory parts of the independent claims for stabilising the web run in a paper machine or the like.
The invention relates typically to a method and a device for stabilising the web run in a paper machine or the like where the web is heated and/or dried by gas infras or the like. In these devices blow suction modules are arranged in connection with the gas infras, typically between the gas infras, in order to stabilise the web run, to improve the runnability, to make the heat and mass transfer more effective, to prevent the web from touching the gas infra structures, to utilise the exhaust gases of the gas infra for drying and for stabilising the web run, and/or to remove combustion gas from the web area.
A typical blow suction module or box comprises
a blow nozzle unit, such as a Float or Foil nozzle of the applicant, which has at least one gap nozzle extending mainly across the web or a corresponding nozzle arrangement extending mainly across the web,
a first air discharge unit extending mainly across the web and being arranged on the front side of the blow nozzle unit, as seen in the travel direction of the web, and having a suction nozzle extending mainly across the web or some other corresponding suction arrangement extending across the web, and
a second air discharge unit extending mainly across the web and being arranged on the back side of the blow nozzle unit, as seen in the travel direction of the web, and having a suction nozzle extending mainly across the web or some other corresponding suction arrangement extending across the web.
The blow nozzle unit or units of the blow suction module blow a gas against the surface of the web, such as combustion gas sucked from the gas infras adjacent to the blow suction unit, air sucked from the web area, replacement air, or other air or gas. Typically the blow nozzle unit comprises a nozzle surface which is parallel to the web and in the connection of which there are arranged the actual blow nozzles, such as one or more gap nozzles or a row of nozzle openings extending across the web, from which nozzles gas can be blown against the web in a desired manner.
The air discharge unit or units of the blow suction module are arranged to suck gas, such as air and/or gas discharged from the gas infras, from the region between the air discharge unit and the web. The air discharge units comprise typically a bottom plate or a corresponding surface, to which there are arranged one or more suction gaps or suction nozzles, such as suction openings.
The suction zones on each side of the blow nozzle unit are thus mainly arranged to suck back air which is blown from the nozzle, and combustion gas which is blown from the adjacent gas infra or the like, and/or flushing gases used in the infra, which thus can be returned to be utilised again. Further the blow nozzle unit sucks air transported by the web.
Disadvantageous blowing and suction arrangements can cause flutter in the web and thus impair the runnability. The fluttering may cause the web to touch the gas infra, which easily results in a web break or which may cause a danger of fire.
The gas infras further tend to react on the air flow created between them and the web, in which case the air flow easily interferes with the function of the radiator in the gas infra. The air flows hitting the radiator element decrease the efficiency of the radiator and they may even extinguish the flame of the infra radiator.
The object of the present invention is to provide an improvement regarding the above presented problems in a paper machine or the like.
One object of the invention is then to optimise the flow field in the region of the blow suction modules so that the creation of detrimental flows is prevented.
It is particularly an object to provide a blow suction module with which the runnability can be improved by minimising the reasons for a flutter created in the above described way.
An object of the invention is also to improve the efficiency of gas infras or the like used in paper machines or the like, by minimising the air flows hitting the radiators.
In order to attain the above mentioned objects the method and the device according to the invention are characterised in that what is defined in the characterising clauses of the independent claims presented below.
Now it has surprisingly been found that
in a dryer which uses symmetric blow suction modules, i.e. modules where both air discharge units suck equal amounts of gas from between the module and the web, there is easily created vibration or flutter in the web, which impairs the runnability and decreases the effect of the gas infra, whereas
in a dryer which uses asymmetric blow suction modules, i.e. modules having a air discharge unit on the front side which sucks more gas than that on the back side, the flutter and vibration can be substantially decreased.
It has been found, that because the air layer transported by the web is discharged mainly on the front side of the blow suction module, it must be possible to remove a sufficient amount of air on the front side. If this is not possible, it can interfere with the function of the module nozzle. On the other hand, the situation is made worse by a too strong air removal at the back side.
The problem becomes bigger when the speed of the web increases. This can be assumed to be due to the fact that when the speed increases the web itself will transport more air with it, which causes a need for an increased asymmetric air removal at the entry and exit sides of the blow suction module.
In the solution according to the invention the aim is to optimise the flow field around the blow nozzle with the aid of an asymmetric control or structure of the air discharge units, so that the air flows disturbing the web run are minimised and an optimal runnability is achieved. This is particularly useful, especially at high web speeds.
A nozzle module according to the invention can be particularly advantageously used in connection with an infra dryer as a system which stabilises the web and effectivates the evaporation. The solution according to the invention enables the use of over-pressure nozzles in the gas infra drying in order to improve the runnability and to effectivate the heat and mass transfer. Then there are suction zones on both sides of the nozzles, through which zones both air blown from the over-pressure nozzles and combustion gas of the infra radiator and/or flushing air is sucked back. According to the invention these gases and air flows can be sucked back smoothly without causing problems. In this way the web flutter is avoided, and in addition also harmful air flows directed at the infra radiator are avoided.
The suction is typically arranged to be asymmetric, so that the larger part of the sucked air and/or gas is discharged at the entry side of the blow suction module, and the smaller part at the exit side.
An asymmetric suction can be provided for instance so that the air discharge unit or the suction zone of the blow suction module is smaller at the exit side than at the entry side of the module. An asymmetric suction can be provided also by control dampers, with which the negative pressure in the different air discharge units can be controlled to have different magnitudes. Then the negative pressure at the entry side is typically controlled to be greater than at the exit side.
The invention can be advantageously used in paper machines or in other corresponding machines, such as in paper board machines, in coating machines, in other finishing machines and in machines for further processing, where the heating and/or drying of the web is made at least partly by gas infras. The invention is particularly well suited to be used in coating machines for heating and drying the coated web immediately after the coating. However, the invention can also be applied otherwise than in connection with gas infras. An asymmetric blow suction module according to the invention can be used e.g. for cooling and evaporation when a good runnability and web stability is required.